Perhaps it might be more logical to suggest other
resources before talking about how to write a post?
That section could also contain a mention of the resources
at this site, or a link to a page that describes it.

I also think that the point on the message body is
incomplete without a link to the page that describes
how to properly format your posts.

As others mentioned, this subject has come up
before (jptxs got the other link I remembered,
but I'm sure there are more).
I'm not really sure that an additional 'line of defense'
is needed because those that want to read the docs will
read them, and those that don't want to won't. I could
see a document like this being integrated with existing
documentation to make a clearer introduction, however.
Maybe you should consider trying to join the site
documentation clan if you have an interest in this sort
of thing? (Note that I do not belong to that group, so
I have no idea if this is a valid suggestion or not--hence
the 'try').

In response to Maclir's suggestion,
I'm opposed to the idea of requiring people to register
in order to use the site properly. I think people are
entitled to privacy if they want it, and right now I
don't think poorly written anon posts are a big enough
problem to warrant such an extreme solution.
I'm sure this, too,
has been discussed before, but I can't remember when.